One of the primary tenants of the federal regulations governing the use of laboratory animals in research is reduction or elimination of unnecessary stress and distress. The practical application of this principle has profound implications to the research community. Not only can stress and distress negatively impact animal welfare but also the physiological perturbations induced by even mild stressors can interfere with the interpretation of research results reducing the productivity of biomedical research. To properly comply with and apply the principle of stress reduction, scientists must determine those factors that research animals commonly experience which induce levels of stress that are either detrimental to their health and well being or, if not detrimental to their welfare, influence experimental results. Effective methods to reduce, eliminate, or at least control these factors in experimental protocols can then be developed. Consequently, the long term goals of the studies in this proposal are: 1) to extend previous and current investigations that have described the effects of some common husbandry and experimental procedures that induce acute stress-like responses of the cardiovascular system, to the characterization of the effects of suspected chronic stressors and to rat strains that are more stress-prone; and 2) to determine how modifications to the animals' physical and social environment can reduce or eliminate stress. To achieve these goals, the following Specific Aims will be addressed: 1) Determine the effects of significantly reduced light intensity and/or a prolonged dark period in animal rooms on cardiovascular, endocrine, and behavioral responses of Sprague-Dawley (SD) and Spontaneously Hypertensive (SHR) male and female rats to suspected or known acute and chronic stressors. 2) Determine the effects of enriching the physical and social environment on cardiovascular, endocrine, and behavioral responses of SD and SHR male and female rats following known or suspected acute and chronic stressors. Cardiovascular measures (blood pressures and heart rate) will be monitored continuously with radiotelemetry to indirectly gauge the activity of the autonomic nervous system; hormones known to be released in response to stressors (prolactin and corticosterone) will be analyzed in the serum of blood samples taken by indwelling catheters and stress related behaviors will be determined by visual observation and standardized behavioral testing. The results should be significant to research investigators and to the federal regulatory agencies that develop animal welfare policies.